在spring mvc和hibernate jpa中解决列表中的空指针异常

Question

我正在使用 spring 和 hibernate jpa。我从数据库中检索了记录列表。在控制器中时，我可以使用 System.out.println() 查看列表的内容。但是当我尝试将此列表传递给另一个方法时，它会显示空指针异常。以下是错误

18-Nov-2015 18:50:47.092 INFO [http-apr-8080-exec-117]org.apache.catalina.core.ApplicationContext.log Destroying Spring    FrameworkServlet 'dispatcher'
18-Nov-2015 18:50:51.724 INFO [http-apr-8080-exec-117]  org.apache.catalina.core.ApplicationContext.log Spring WebApplicationInitializers detected on classpath:  [com.domkat.springmvcjpa.config.AppInitializer@18a550e]
18-Nov-2015 18:50:52.168 INFO [http-apr-8080-exec-117] org.apache.catalina.core.ApplicationContext.log Initializing Spring FrameworkServlet 'dispatcher'
18-Nov-2015 18:51:14.761 SEVERE [http-apr-8080-exec-116] org.apache.catalina.core.StandardWrapperValve.invoke Servlet.service() for servlet [dispatcher] in context with path [/mysurvey] threw exception [Request processing failed; nested exception is java.lang.IndexOutOfBoundsException: Index: 0, Size: 0] with root cause
java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
at java.util.ArrayList.rangeCheck(ArrayList.java:638)
at java.util.ArrayList.get(ArrayList.java:414)
at  org.hibernate.collection.internal.PersistentBag.get(PersistentBag.java:440)
at com.domkat.springmvcjpa.processor.Processor.traverseLegsBearings(Processor.java:271)
at com.domkat.springmvcjpa.controller.StationController.analyseSurvey(StationController.java:132)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at org.springframework.web.method.support.InvocableHandlerMethod.invoke(InvocableHandlerMethod.java:214)
at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:132)
at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:104)
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:749)
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:690)
at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:83)
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:945)
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:876)
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:961)
at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:863)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:648)
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:837)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:291)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:217)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:142)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:616)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:518)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1091)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:673)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2503)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2492)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:745)

请帮忙。 下面是控制器方法

 @RequestMapping(value="sanalyse", method=RequestMethod.POST)
public String analyseSurvey(Model model){
    List<Fromtost>ftcList=ftss.getFromtostList();
    model.addAttribute("fromtostations", ftcList);
    System.out.println(ftcList.size());
    Processor processor=new Processor();
    processor.traverseLegsBearings(ftss.getFromtostList());

return "tables";
}

下面是我尝试传递给它的方法

public void traverseLegsBearings(List<Fromtost> ftcList) {


    double backwardBearingOfLegs = 0.0;
    double cpBB = ftcList.get(0).getSurveyId().getFromtocpsList().get(0).getBb();
    double firstOAdecDeg = ftcList.get(0).getDegdec();
    double numberOfStations = ftcList.size();
    double forwardBearingOfLegs = 0.0;
    List<ArrayList> bearings = new ArrayList<>();

    for (Fromtost s : ftcList) {

        forwardBearingOfLegs = (double) cpBB + s.getDegdec();
        if (forwardBearingOfLegs >= 360) {
            forwardBearingOfLegs = forwardBearingOfLegs - 360;
            //forwardBearings.add(forwardBearingOfLegs);
        }

        if (forwardBearingOfLegs >= 180) {
            backwardBearingOfLegs = forwardBearingOfLegs - 180;

        } else {
            backwardBearingOfLegs = forwardBearingOfLegs + 180;


        }

        s.setFb(forwardBearingOfLegs);
        s.setBb(backwardBearingOfLegs);

        forwardBearings.add(forwardBearingOfLegs);
        backwardBearings.add(backwardBearingOfLegs);
        cpBB = backwardBearingOfLegs;

    }

    /*bearings.add(forwardBearings);
    bearings.add(backwardBearings);*/



}

谢谢。

控制器

@Controller
public class StationController {
@Autowired
private FromtocpsService ftcs;
@Autowired
private FromtostService ftss;
@Autowired
private SurveysService ss;


@RequestMapping(value="sanalyse", method=RequestMethod.POST)
public String analyseSurvey(Model model){
    List<Fromtost>ftcList=ftss.getFromtostList();
    model.addAttribute("fromtostations", ftcList);
    System.out.println(ftcList.size());
    Processor processor=new Processor();
    processor.traverseLegsBearings(ftss.getFromtostList());

return "tables";
}

所涉及的实体是 调查：

public class Surveys implements Serializable {
private static final long serialVersionUID = 1L;
@Column(name = "surveytitle")
private String surveytitle;
@Column(name = "numofstations")
private Integer numofstations;
// @Max(value=?)  @Min(value=?)//if you know range of your decimal fields consider using these annotations to enforce field validation
@Column(name = "g")
private Double g;
@Column(name = "n")
private Double n;
@Column(name = "misclosureineastings")
private Double misclosureineastings;
@Column(name = "misclosureinnorthings")
private Double misclosureinnorthings;
@Column(name = "bearingmisclosure")
private Double bearingmisclosure;
@Column(name = "allowablemisclosure")
private Double allowablemisclosure;
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Basic(optional = false)
@Column(name = "survey_id")
private Integer surveyId;
@Column(name = "matNumber")
private String matNumber;
@OneToMany(cascade=CascadeType.ALL,mappedBy = "surveyId",fetch=FetchType.EAGER)
@Fetch(value = FetchMode.SUBSELECT)
private List<Fromtocps> fromtocpsList;
@OneToMany(cascade=CascadeType.ALL,mappedBy = "surveyId",fetch=FetchType.EAGER)
private List<Fromtost> fromtostList;

2)Fromtocps

public class Fromtocps implements Serializable {
private static final long serialVersionUID = 1L;
// @Max(value=?)  @Min(value=?)//if you know range of your decimal fields consider using these annotations to enforce field validation
@Column(name = "distance")
private Double distance;
@Column(name = "fl1")
private String fl1;
@Column(name = "fl2")
private String fl2;
@Column(name = "fr1")
private String fr1;
@Column(name = "fr2")
private String fr2;
@Column(name = "diffleft")
private String diffleft;
@Column(name = "diffright")
private String diffright;
@Column(name = "meandiff")
private String meandiff;
@Column(name = "decdeg")
private Double decdeg;
@Column(name = "eastingschange")
private Double eastingschange;
@Column(name = "northingschange")
private Double northingschange;
@Column(name = "fb")
private Double fb;
@Column(name = "bb")
private Double bb;
@Column(name = "fromtolabel")
private String fromtolabel;
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Basic(optional = false)
@Column(name = "fromtocps_id")
private Integer fromtocpsId;
@JoinColumn(name = "surveyId", referencedColumnName = "survey_id")
@ManyToOne
private Surveys surveyId;
@JoinColumn(name = "fromtost_id", referencedColumnName = "fromtost_id")
@ManyToOne
private Fromtost fromtostId;
@OneToMany(cascade=CascadeType.ALL,mappedBy = "fromtocpsId")
private List<Tocp> tocpList;
@OneToMany(cascade=CascadeType.ALL,mappedBy = "fromtocpsId")
private List<Fromcp> fromcpList;

Answer 1

ftcList.get(0).getSurveyId().getFromtocpsList().get(0).getBb();

如果您的列表为空，您不能在此处get(0)。您应该始终清理您的输入。

if ( ftcList != null && !ftcList.isEmpty() ) {
    // Your code here
}

getFromtocpsList() 也是如此